Generally, for a plurality of transmission stations mutually connected in turn in the form of a ring through communication lines capable of bi-directional communications to constitute a network system, the transmission stations are each adapted to send transmission frames in both directions.
Those transmission stations are each adapted for one of preset transmission stations to work as a control station, so that transmission frames sent from respective transmission stations are kept from continuing circulation in the ring.
As a conventional example of this system cutting a transmission frame flow at a transmission station, we have Japanese Patent Publication No. 3,461,954 (patent document 1).
In this Patent document 1, if a trouble happens at any single location while running, this transmission station operates as a central control station for interactions with transmission stations in a network, mainly in particular with those transmission stations which have detected the trouble, to cut off a part in trouble.
Or, having a trouble determined from a break in a steady intercommunication by transmission frame, it cuts off a part in trouble, and implements a bi-directional repeating of transmission frame that it has cut till then as a control station, which is done as a system to avoid an entire shutdown of network system due to one-point trouble.
There is also an IEEE 802.5 token ring system network, as a similar system.
On the other hand, there is an ANSI X3T 9.5 FDDI as a ring form network, in which transmission stations are normally each adapted to send transmission frames in a direction on a ring, and a respective transmission station is adapted to repeat transmission frames sent from other transmission stations, and any transmission frame that has come around through the ring is dropped at the station where it has been sent, thereby keeping the transmission frame from continuing circulation in the ring.
This type of network has communication lines mutually permitting a bi-directional communication, and normally employs a communication line for one direction, with a communication line for the other direction held in a waiting state.
If a trouble happens at any single location while running, transmission frames are to be blocked there, whereto a predetermined central transmission station interacts mainly with transmission devices adjacent to the location of trouble, to have a communication line fold back at the adjacent paired transmission stations with a troubled part in between, so a reverse directional communication line that has been in a waiting state till then revives to constitute a new ring network, while having a communication line doubled in length. It is thereby avoided to undergo an entire shutdown of network due to one-point trouble.
Like this, it is typical for examples in the past to block a transmission flow at a stationary single point in a ring form network.
The point to be blocked is at a central control station fixedly determined in advance, or such a transmission station that is temporarily given a transmission right to send transmission frames on a common transmission line, as it can stand as a control station to control the transmission line, while holding the transmission right.
Further, there is disclosure (patent document 2) of a data transmission processing system in which an application program for a respective data transmission device is sent from a single point, and respective transmission devices are remote-controlled for maintenance from the single point, allowing for an improved maintenance work efficiency and work saving.    Patent document 1: Japanese Patent Publication No. 3,461,954    Patent document 2: Japanese Patent Application Laid-Open Publication No. 5-289968